Amplification of brightness variability by active-region nesting in solar-like stars

TL;DR

This study suggests that increased active-region emergence and nesting on solar-like stars can explain their stronger, more regular brightness variations compared to the Sun, highlighting the role of active longitudes.

Contribution

The paper introduces a model demonstrating how higher emergence rates and nesting patterns of active regions can account for observed stellar brightness variability.

Findings

Increased AR emergence frequency explains variability amplitudes.

Nesting leads to regular, sine-like brightness patterns.

Active longitudes cause highly regular variability.

Abstract

Kepler observations revealed that hundreds of stars with near-solar fundamental parameters and rotation periods have much stronger and more regular brightness variations than the Sun. Here we identify one possible reason for the peculiar behaviour of these stars. Inspired by solar nests of activity, we assume that the degree of inhomogeneity of active-region (AR) emergence on such stars is higher than on the Sun. To test our hypothesis, we model stellar light curves by injecting ARs consisting of spots and faculae on stellar surfaces at various rates and nesting patterns, using solar AR properties and differential rotation. We show that a moderate increase of the emergence frequency from the solar value combined with the increase of the degree of nesting can explain the full range of observed amplitudes of variability of Sun-like stars with nearly the solar rotation period. Furthermore, nesting in the form of active longitudes, in which ARs tend to emerge in the vicinity of two longitudes separated by $180^\circ$, leads to highly regular, almost sine-like variability patterns, rather similar to those observed in a number of solar-like stars.